1. Field of the Invention
The present invention relates generally to a control system for controlling operation of a limited-slip differential gear unit. More specifically, the invention relates to a limited-slid differential gear unit control system cooperative with a wheel-spinning preventive drive control system for adjusting driving torque to be applied to driven wheel of the vehicle.
2. Description of the Background Art
Conventional limited-slip differential gear unit has been disclosed in "Jidosha Kogaku Zensho" Vol. No. 9, "Power Train Device", published on Nov. 15, 1980, by Kabushiki Kaisha Sankaido, on pages 321 to 324, for example.
Such conventional limited-slip differential gear unit employs a slip-limiting mechanism which comprises multi-plate friction clutches. The multiplate friction clutches are interposed between a differential casing and side gears. For the multiplate friction clutch, a thrusting force generated by a cam mechanism provided on a pinion mate shaft depending upon driving torque, is applied as a clutch engaging force. According to the clutch engaging force thus generated, slip-limiting torque is generated by the slip-limiting mechanism. Such type of slip-limiting mechanism has been known as "torque-proportional type slip-limiting mechanism".
On the other hand, traction control systems have been developed for adjusting driving torque to be applied to driven wheels for preventing the driven wheels from causing wheel-spinning in order to optimize vehicular performance. In general, wheel slippage is reflected in a difference of an actual vehicle speed and an assumed vehicle speed derived based on a wheel speed. When the actual vehicle speed is lower than the assumed vehicle speed, it means the driven wheel is caused to spin. On the other hand, when the actual vehicle speed is higher than the assumed vehicle speed, it means the vehicle wheel is skidding. Wheel-spinning is caused by loss of road/tire traction. Therefore, in such a case, traction control has to be performed in order to prevent the vehicular wheel from spinning. On the other hand, wheel-skidding occurs during application of abrupt braking and is caused by locking of the wheel. For example, such traction controls have been disclosed in U.S. Pat. No. 3,893,535, issued on July 8, 1975, to M. H. Burckhardt, et al. and in Japanese Patent First Publication (Tokkai Showa) 59-68537, published on Apr. 18, 1984. In both cases, a rotation speed of a driven wheel which is driven by an engine output is compared with a rotation speed of a non-driven wheel which rotates freely. The rotation speed of the non-driven wheel is treated as a parameter reflecting an actual vehicle speed.
Such conventional limited-slip differential gear units and the the traction control systems can provide better vehicular driving performance when they are observed independently. For instance, the limited-slip differential gear unit may adjust distribution of driving torque between higher load wheel and lower load wheel by limiting slip in the differential gear unit so that driving torque distribution should not be too concentrated to the low load wheel. On the other hand, the traction control system may provide optimum driving torque for driven wheels in order to maintain driving stability and road/tire traction at reasonable level. As will be appreciated better drivability may be obtained by cooperating the slip control system for limited-slip differential gear unit and the traction control system set forth above.
Such slip control system for a limited-slip differential gear unit cooperated with drive torque control system has also been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) No. 60-143135, published on July 29, 1985. In the disclosed system, the limited-slip differential gear control system is cooperated with a traction control system. The slip control for the limited-slip differential gear unit becomes active for establishing locking of the differential gear unit for limiting slip therein for adjustment of the driving torque distribution in such vehicle driving condition that there is high possibility of wheel-spinning due to excessive driving torque to be exerted on driven wheels. On the other hand, the slip control for the differential gear unit is deactivated when steering angular displacement is greater than a given angle or steering behavior causes lateral force greater than a given magnitude, or the difference of rotation speeds of driving torque becomes greater than a given value, throttle valve angular position is fully closed or near fully closed position, or the brake is applied.
As will be appreciated herefrom, the system disclosed in the Japanese Tokkai Showa 60-143135 cannot provide sufficient force reactive on the lateral force which is created during cornering or turning of the vehicle, by limiting slip in the differential gear unit for increasing driving torque to be distributed to the driven wheels. Namely, in the disclosed system, since the slip limitation of the differential gear unit is not active while the vehicle is curving or turning by operating steering at an angle greater than the given angle the distribution rate of driving torque of the engine to the inner and lighter load wheel is increased. The traction control becomes active at this condition due to a greater difference of the wheel speeds between the driven wheels than a given value. Therefore, the driving torque to be distributed to the driven wheel may be further reduced to degrade vehicular acceleration characteristics. On the other hand, even when the slip-limited differential gear unit is locked during cornering or turning, the slip ratio at the outer driven wheel will become greater than that in an unlocking condition, since the traction control will never be initiated.